In the field of food packaging, increased usage has been observed for modes of packaging such as cups and trays filled with contents such as instant noodles, jelly, or yogurt. Such cups or trays are generally manufactured using a method such as injection molding or vacuum compression molding a thermoplastic resin such as polystyrene, expanded polystyrene, and polypropylene resins.
With regard to the qualities required for a cover material used in a cup container, a sealant layer which is the innermost layer of the cover material for protecting a content is preferably strongly adhered to the container such as a cup, which is an adherent. On the other hand, an opposite quality of being easily unsealable only upon unsealing is demanded.
In order to satisfy the required qualities described above, various functions have been added to the cover material. One of such functions involves a design of an easily unsealable mechanism. Examples of the easily unsealable mechanism include an inter-layer peeling mechanism and a cohesion peeling mechanism. Among these, the cohesion peeling mechanism is designed by blending, in the thermoplastic resin, a non-compatible type or a partially compatible type thermoplastic resin. The peeling principle behind the cohesion peeling mechanism relates to making use of the small cohesive force of the non-compatible type or the partially compatible type thermoplastic resin blend layer and utilizing a cohesion failure in the non-compatible type or the partially compatible type thermoplastic resin blend layer, instead of rupturing at an interface between the adherent and the sealant at the innermost part of the cover material upon unsealing.
Problems of the cohesion peeling mechanism described above include occurrence of stringiness upon unsealing. Occurrence of stringiness is suppressed by reducing layer thickness of an easily unsealable layer (thermoplastic resin blend layer) as much as possible. Alternatively, since the easily unsealable layer generally forms a sea-island structure in which a fine spherical domain phase is dispersed in a matrix phase, it is also possible to suppress occurrence of stringiness through morphological designing such as reducing the dispersion diameter of the domain phase as much as possible. However, thinly forming the thermoplastic resin blend layer is an extremely difficult technology in terms of film thickness control, resulting in deteriorated processability. Furthermore, since finely dispersing the domain phase significantly improves interface adhesiveness of two members, unsealing strength becomes strong and it is difficult to provide the ease of unsealing.